After looking at some pics from Cass, I got to wondering, how much superheat can slide valves handle before warping sets in? Or does it? Is it a function of size or design?

I remember hearing Corliss rotary engine valves did not like too high a pressure or superheat.

I don't have a quantified answer to your question - but it is in the back of my mind that superheat wasn't widely accepted until several other bits of technology came along to make it reliable, namely piston valves, Walschaerts gear, high-temp steam oils, and mechanical force feed lubrication.

As I understand it, the size of the inside Stephenson gear necessary to force the slide valve became onerous to fit between the frames, much less hook up, so a more balanced piston valve was necessary, as well as a valve gear that was free to grow without the inside of the frames limitation. Better oils delivered more reliably is more obviously understood given the higher temperatures. I don't know if metallic packings became popular during this transition also, but wouldn't be surprised.

Of course, none of this would have been possible without steel boilers replacing wrought iron, but that's another story.

dave

It's interesting on how one finds a mixture of older and newer technologies in steam locomotive development.

Take for example Terminal Railroad Association of St. Louis 0-8-0 318, at MOT in St. Louis. It was the first, or one of the first steam locomotives with the 1 piece cast frame, made by GSI over in Granite City. Even with the modern cast frame, it still sports a wood cab, and I think a hydrostatic lubricator (maybe I'm looking on the wrong side of the locomotive). At least it has power reverse....

The limitations of slide valves are more a function of boiler pressure rather than superheat temperature. The force which the slide valve exerts on its seat is the area of the slide valve times the boiler pressure. The force required to move the slide valve is a function of the friction coefficient and the sealing force on the slide valve. On a piston valve, the force required to move the piston valve is generally independent of the steam pressure. The size of the slide valve is a function of the swept area of the cylinder.

For the pressures at which slide valves can operate, greater increases in power and efficiency are realized by an increase in pressure rather than the use of superheat. Hence the transition to piston valves.

As material temperature limitations were reached (around 300 psi, or about 525F for saturated steam) the use of superheat was then the only available route for increases in power and efficiency. Materials were available at the time which could withstand the additional superheat temperatures ( an additional 100F to 200F) and provide satisfactory service.

The other limitation to the amount of superheat is the temperature limitations of the lubricant.

Hope this helps.

pkurilecz

In the day there was a product called a "Franklin Economy Steam Chest" which converted a slide valve engine to an outside admission spool valve. There were numerous Boston and Maine Moguls converted in this manner. The 1455 which survives has such an example. This was probably done so the engines could be superheated.
LF

The Franklin Universal Valve Chest advertising:

"A successful piston valve arrangement for application to existing slide valve engines. It saves the old slide valve cylinders and frame ends. Superheating can be accomplished in the shortest possible time because much hard-to-get material and labor is eliminated."

MX

With this conversion from slide valves to piston valves, it appears that they also add an outside admission pipe. Does this pipe enter the center of the valve chest top surface?

If so, is the top cover of the valve chest split so it can be removed or applied without disconnecting the admission pipe?

RTK

Ron, that is correct and the top cover split line runs front to back (making inboard and outboard cover halves) lined up through the center of the admission pipe.

MX

Thanks MX. I am working on a painting from several photo references, but I could not see that detail. For me, it is quite a coincidence that this superheater conversion topic came up because I have been wrestling with images of it for the last couple days, and thought about posting the question.

RTK

Just to clarify for those who have not touched slide valve, the seat force is generated by the exposed area outside of the balancing bars. These leafspring-loaded bars are intended, though not perfectly, keep live steam off the majority of the area of the valve. Any leakage is communicated to atmosphere through the exhaust port through hole(s) in the top center. The higher temps would have an effect on the springs' effectiveness at elevated temps that come with higher pressures.

The IRCA Baldwin 2-8-0's - old Georgetown Loop 40 and 44, future Georgetown Loop 111 - are equipped with the piston valve conversion kits, which George Sapp informs me could be bought from the J C Whitney catalogs back in the early 20th century. I have only worked on one of these in steam - and it was a beast to hook up. The valve acted like a piston and wanted to throw you out of the cab either forwards or backwards the instant you tripped the latch. Might have had something to do with the Walschaerts gear conversion installed during the upgrading / conversion. I wish they had also included screw reverse!

44 is torn down to the frame in Golden - easy to see how it is installed if you visit there.

dave

Nevada Northern was a user of the Franklin Piston Valve conversion. Both #40 and #93 have conversions. They had a few others too.

On the Sierra 18 (while sitting tenderless and derelict at Jamestown):

http://photos.nerail.org/showpic/?photo ... ate&page=2

B&M 1455 shows only one pipe going to the converted steam chest. I couldn't remember what she looked like after not being there for many years.

I don't know much about locomotives using superheat and slide valves but Stanley Steamers used slide valves with superheat. The engines used D slide valves with Stevenson linkage. Normal operation of a Stanley is from 400-600 psi and 450-600 degree F. Slide valve wear is reasonable if the engine is not allowed to run without oil.

Alan